1. Field of the Invention
The present invention relates to a balanced acoustic wave filter device having a balanced-unbalanced conversion function.
2. Description of the Related Art
At a front end of a mobile communication apparatus, a surface acoustic wave filter is connected between an antenna and a differential amplifier to define a bandpass filter. The antenna inputs and outputs unbalanced signals while the differential amplifier inputs and outputs balanced signals. Accordingly, a component having a balanced-unbalanced conversion function must be connected between the antenna and the differential amplifier. If the surface acoustic wave filter has a balanced-unbalanced conversion function, the need for another component having a balanced-unbalanced conversion function, such as a balun, is eliminated.
The characteristic impedance of the antenna is about 50Ω while the characteristic impedance of the differential amplifier is greater than or equal to about 100Ω, and in some situations, it reaches about 1,000Ω. Therefore, between the antenna and the differential amplifier, impedance conversion is required in addition to balanced-unbalanced conversion. Consequently, it is desirable that a surface acoustic wave filter having the balanced-unbalanced conversion also has the impedance conversion function.
Japanese Unexamined Patent Application Publication No. 2003-69383 (Patent Document 1) described below discloses a surface acoustic wave filter device shown in FIG. 16. A surface acoustic wave filter device 201 includes electrodes having a structure shown in FIG. 16 that are provided on a piezoelectric substrate 202. The surface acoustic wave filter device 201 includes an unbalanced terminal 203 and first and second balanced terminals 204 and 205. First to third IDTs 211 to 213 are arranged in a surface wave propagating direction. Reflectors 214 and 215 are arranged on either side of an area in which the IDTs 211 to 213 are arranged in a surface wave propagating direction.
The second middle IDT 212 is divided into two pieces in a surface wave propagating direction so as to include first and second divided IDT portions 212a and 212b. The unbalanced terminal 203 is connected to one end of the first IDT 211 and one end of the third IDT 213, while the other end of the IDT 211 and the other end of the IDT 213 are connected to ground. Additionally, the first and second divided IDT portions 212a and 212b are connected to the first and second balanced terminals 204 and 205, respectively.
The above-described surface acoustic wave filter device 201 has a balanced-unbalanced conversion function and further has an impedance conversion function. That is, the second IDT 212 is divided into the first and second divided IDT portions 212a and 212b, which are electrically connected in series. Accordingly, the impedance of the second IDT 212 is four times the impedance of an IDT before being divided into the first and second divided IDT portions 212a and 212b. Accordingly, in the surface acoustic wave filter device 201, the impedance ratio of the unbalanced terminal 203 to the balanced terminal 204 or 205 is about 1:4.
As described above, it is strongly desired for a surface acoustic wave filter connected between an antenna and a differential amplifier to have an impedance conversion function in addition to a balanced-unbalanced conversion function. As stated above, the surface acoustic wave filter device 201 discussed in Patent Document 1 has an impedance ratio of about 1:4 between the unbalanced terminal 203 and the balanced terminal 204 or 205. Therefore, the surface acoustic wave filter device 201 has an approximately quadruple impedance conversion function.
However, nowadays, it is not uncommon that the characteristic impedance of the above-described differential amplifier exceeds about 1,000Ω. Accordingly, when, for example, the input and output impedance of an antenna is about 50Ω and the characteristic impedance of a differential amplifier is about 1000Ω, a 1:20 impedance conversion is required. Although the surface acoustic wave filter device 201 discussed in Patent Document 1 can increase the impedance to about four times the original impedance, the surface acoustic wave filter device 201 cannot achieve an impedance conversion function that increases the impedance to more than four times the original impedance.
Accordingly, if an impedance conversion function that increases the impedance to more than four times the original impedance is required, an additional component for impedance conversion is required in addition to the surface acoustic wave filter device 201.
In recent years, a boundary acoustic wave filter device that utilizes boundary acoustic waves has been used as an acoustic wave device in addition to a surface acoustic wave filter device. For other types of acoustic wave filter devices, such as a boundary acoustic wave filter device, a high-ratio impedance conversion function is also desired in addition to a balanced-unbalanced conversion function.